Visualization of the S-locus region in Ipomoea trifida: toward positional cloning of self-incompatibility genes.

Self-incompatibility (SI) in Ipomoea trifida is regulated by a single S locus with multiple alleles. Identification of SI genes in the S -locus region by positional cloning is one of the most important goals for understanding sexual reproduction in this species. Despite our intensive efforts to construct bacterial artificial chromosome (BAC) contigs covering the S -locus region, a gap was observed in the core region of the potential S locus. In order to confirm the physical linkage of two non-overlapping BAC contigs in the S -locus region and to determine the size of the gap between them, fluorescence in-situ hybridization (FISH) was performed on mitotic chromosomes and extended DNA fibres using previously isolated S -linked BAC clones as probes. The information obtained from this work would be useful for molecular cloning of the SI genes by a chromosome walking approach. In addition, we showed that strong suppression of recombination in the S locus was not related to the centromere because the S locus was mapped to one end of a chromosome.